1. Field of the Invention
Broadly, the present invention relates to the production of gaseous hydrogen. More particularly, it relates to a two-step cyclic process which utilizes a halogen for the production of hydrogen from carbonaceous materials and water.
2. Prior Art
The United States presently consumes more energy than it produces. A large portion of the energy consumed is in the form of oil and natural gas; a major portion of the oil being imported. The reserves of oil and natural gas in the United States are being rapidly depleted. Thus, the country is becoming even more dependent upon imported petroleum and may soon have to import significant quantities of natural gas. The present principal uses of hydrogen are for the production of ammonia to make fertilizer, the production of explosives and hydrocracking of hydrocarbonaceous materials. It has been proposed that hydrogen be produced and used as a source of fuel and as a chemical feedstock to reduce the country's dependence on petroleum and natural gas.
The principal source of hydrogen produced in the United States comes from the steam-hydrocarbon process wherein a hydrocarbon feed material is reacted with steam to produce a product gas comprising carbon monoxide, hydrogen and carbon dioxide. The steam-hydrocarbon process is complex and requires that the product gas be treated to remove the carbon monoxide and carbon dioxide to obtain substantially pure hydrogen. In addition, if the hydrocarbon feed material contains sulfur, then the feed material or product gas must be treated to remove the sulfur to provide a non-polluting hydrogen gas product.
A high purity hydrogen product is obtained from the electrolysis of water. The theoretical voltage required to decompose water is reported as being 1.23 volts at atmospheric termperature and pressure. Thus, assuming 100% current efficiency and the theoretical decomposition voltage, the production of 1000 scf of hydrogen would require approximately 78.0 kwhr of direct current. In actual practice, however, the minimum voltage required to produce gaseous hydrogen from a solution of potassium of sodium hydroxide is approximately 1.7 volts, while the actual operation voltage of commercial cells is reported as ordinarily in the range of from about 2.0 to 2.5 volts. In addition, the electrolysis of water requires direct current (dc) and most electrical power is produced and supplied to plants as alternating current (ac). Accordingly, a rectifier must be used to convert the ac to dc; this imposes an additional power loss. Thus, the energy used in commercial cells amounts to about 130 to 160 kwhr per thousand standard cubic feet of hydrogen produced.
A disadvantage of producing hydrogen by the electrolysis of water is that the cost is substantially higher than the steam-hydrocarbon process.
Recently there has been proposed a thermochemical hydrogen process which uses calcium bromide and mercury to decompose water. The temperature required is about 727.degree. C, which is a temperature attainable in the steam discharged from a high temperature gas reactor. A disadvantage of this process however, is the high cost of volatility of mercury. Specifically, the loss of significant amounts of mercury to the atmosphere would appear to be certain to occur in the course of continuous operation, adding to the expense of the process and creating a potentially severe ecological hazard.
U.S. Pat. No. 3,839,550 discloses a cyclic process for the production of hydrogen from water. The process comprises the steps of hydrolyzing lithium iodide to produce lithium hydroxide and hydriodic acid in a reaction zone and removing the hydriodic acid from the reaction zone; the remaining aqueous lithium hydroxide is then reacted with iodine to produce an aqueous mixture of lithium iodide and lithium iodate. The lithium iodate is separated from the lithium iodide and reacted in the presence of water with a metal selected from the group consisting of potassium, rubidium and cesium to form a selected metal iodate. The selected metal iodate is thermally decomposed to produce oxygen and the selected metal iodide, the latter of which is recycled for reaction with additional lithium iodate. The hydriodic acid is converted to hydrogen and iodine, the hydrogen being continuously removed as the product, and the iodine being recycled for reaction with aqueous lithium hydroxide. This process is somewhat complex and has not yet been proven to be commercially economic.
A process for producing hydrogen from the disassociation of a hydrogen halide is disclosed in U.S. Pat. No. 3,365,276. Broadly, the claimed process comprises heating a hydrogen halide to a temperature above 600.degree. F in a first zone in the presence of a hydrogen permeable membrane to disassociate the hydrogen and the halogen. A pressure differential is maintained across the hydrogen permeable membrane such that the hydrogen produced passes through the hydrogen permeable membrane for recovery. A disadvantage to this process is that it requires a substantial amount of thermal energy to effect the disassociation of the hydrogen halide.